1. Field of the Invention
The invention relates to an apparatus for feature detection of a test object, more particularly to a simple and low cost apparatus for feature detection of a test object.
2. Description of the Related Art
Automated optical inspection is used during the manufacture of printed circuit boards and the like. During an automated surface mounting process, an automated optical inspection system is used to verify features of mounted electric components. A conventional automated optical inspection system generally includes an image capturing device to be disposed above a target printed circuit board and operable so as to capture images of test regions of the target printed circuit board, a moving device for generating relative movement between the image capturing device and the target printed circuit board, and a computerized control device for controlling the operation of the moving device and for analyzing the images captured by the image capturing device so as to verify the features of the test regions of the target printed circuit board. The image capturing device used in automated optical inspection systems usually includes a light source module and an image capturing module.
Features of printed circuit boards, which require verification, include edges of electric components, metal traces, and solder contacts. It is noted that these features can be simultaneously captured in an image only when light from the light source module illuminates the printed circuit board at a certain angle relative to the image capturing module. For example, if an electric component on the printed circuit board has a shiny surface that reflects light away from the image capturing module, the image capturing module would not be able to receive the reflected light from the electric component, and the area where the electric component is supposed to be in would appear black in the images captured by the image capturing module, thereby preventing the control device from successfully verifying the features of the printed circuit board.
Therefore, to create an image that is useful for automated inspection, illumination of the test regions by the light source module must be controlled relative to the image capturing module. To achieve this purpose, the light source module is generally designed to illuminate a test region of the printed circuit board from different angles, or to illuminate the test region from different combinations of angles.
In WO 02/01210 A1, there is disclosed an optical inspection system with an illumination system that includes substrates having serrations, and lighting elements (such as diodes that exhibit good beam radiation patterns) mounted on the serrations. The light emitting elements are focused on a focal point, and have different beam widths so that variations in illumination intensity as a function of elevation angle are reduced.
While the illumination system in WO 02/01210 A1 permits illumination of a test region from different angles or from different combinations of angles, formation of the serrations involves complex and precise manufacturing processes and equipment, which result in high production costs. In addition, such an illumination system cannot be flexibly configured.